Production of electrophotographic paper by electrophoretic deposition

ABSTRACT

ELECTROPHOTOGRAPHIC PAPER IS PROVIDED BY ELECTROPHORETICALLY DEPOSITING ON PAPER ZINC OXIDE AND A RESINOUS BINDER ASSOCIATED THEREWITH FROM NONAQUEOUS MEDIUM AT THE CATHODE OF A UNIDIRECTIONAL ELECTRICAL SYSTEM.

y 2, 1972 L- L. SPILLER 3,550,262

PRQDUCTION OF ELECTKUPHOTQGRAPHIC PAPER BY ELECTRQPHORETIC DEPOSITIONFiled June 19, 1969 O O P/IPER CW 1 za EA 52w? SURFACE flCT/VE nae/v7ELECTRO C04 T/NG BHTH ' INVENTOR LESTER L. SP/LLER United States Patentice 3,660,262 Patented May 2, 1972 3,660,262 PRODUCTION OFELECTROPHOTOGRAPHIC PAPER BY ELECTROPHORETIC DEPOSITION Lester L.Spiller, Indianapolis, Ind., assignor to Ransburg Electro-Coafing Corp.,Indianapolis, Ind. Filed June 19, 1969, Ser. No. 834,815 Int. Cl. B011:/02

US. Cl. 204-181 14 Claims ABSTRACT OF THE DISCLOSURE Electrophotographicpaper is provided by electrophoretically depositing on paper zinc oxideand a resinous binder associated therewith from nonaqueous medium at thecathode of a unidirectional electrical system.

The present invention relates to the production of electrophotographicpaper for use in electrostatic copying processes by the electrophoreticdeposition of z1nc oxide and a resinous binder associated therewith fromnonaqueous medium on to the paper base immersed therein supported on thecathode of a unidirectional electrical system.

The production of zinc oxide-coated electrophotographic paper using zincoxide-pigmented resin coatings is well known and the problems connectedtherewith are also well known.

Zinc oxide is heavy so that excessive coating material addsunnecessarily to the cost and weight of the paper. Moreover, anon-uniform deposit is detrimental since it causes the production of abackground pattern when the paper is exposed and developed by depositionof toner particles as is well known in xerography. The normal coatingtechniques are inadequate because the surface of paper is rough and thesurface volume of the paper is considerable and must be filled before auniform surface can be provided. The coating used to fill this surfacevolume is not necessary to performance and only serves to produce afinal coating which is unnecessarily heavy and non-uniform in thickness,being thicker wherever the coating fills a depression in the originalsurface.

The advantage of an electrophoretically deposited coating is itsuniformity in thickness which is independent of any irregularity in thesubstrate which receives the coating. This permits a thinner, lighterand more uniform coating to be provided.

In accordance with the present invention, the paper is renderedconductive as by impregnation with a solution of a surface active agent,preferably a cationic surface active agent, and the conductive paper isadvanced through a nonaqueous electrocoating bath while in the vicinityof the cathode and preferably in direct contact with the cathode of aunidirectional electrical system. In the invention, the electrocoatingbath contains zinc oxide pigment dispersed in an organic solvent mediumin which an oil-free binder resin (preferably an addition polymer) ispresent in large proportion with respect to the pigment. Since the paperis porous and conductive, the electrical system is able to effectivelyoperate through the intervening paper to cause the zinc oxide pigmentand the resin associated therewith in the electrocoating bath to beelectrically propelled toward the cathode and to deposit on the surfaceof the paper providing a smooth and uniform coating Which follows thesurface configuration of the paper which is coated.

Cationic surface active agents are particularly preferred for renderingthe paper conductive, but any conductive agent may be used includinganionic and nonionic agents. While selection of surface active agent toprovide the desired conductivity is a secondary feature of theinvention, suitable anionic agents are illustrated by sulfates such assodium lauryl sulfate and sulfonates such as sodium octylphenoxypolypropylene oxide sulfonate. Nonionic agents are illustrated byoctylphenoxy polyoxypropylene ethanol. Appropriate cationic surfaceactive agents are illustrated by alkyl benzyl triethyl ammoniumchloride. Numerous other surface active agents within the classes notedabove are well known. With reference to the bath containing the surfaceactive agent, a concentration of agent of from 0.1-5.% is appropriate.

A feature of importance in the selection of the surface active agent isits relationship to the solvent medium used in the electrocating bath.It is particularly preferred to employ a surface active agent which isessentially insoluble in the solvent medium of the bath. Thus, selectinga cationic surface active agent such as alkyl benzyl triethyl ammoniumchloride which is poorly soluble in hydrocarbon solvents, this surfaceactive agent can be applied from solution in a solvent medium comprisingxylene enriched with isopropanol (typically 35%) in order to adequatelydissolve the agent which is conveniently used in 1% concentration. Onthe other hand, and especially after the impregnated paper has beendried, the surface active agent is poorly soluble in the substantiallypure aromatic hydrocarbon solvent medium of the electrocoating bath sothat gross contamination of the electrocoating bath with surface activeagent can be avoided or minimized. It is a feature of the invention tofind that electrodeposition can be moved out away from the cathode andonto the surface of the paper without using a soluble antistatic agent.

Accordingly, it is preferred to employ an electrocoating bath whichconsists essentially of hydrocarbon solvent, either aromatic oraliphatic, and with the surface active agent being essentially insolublein such medium, though soluble in stronger solvent systems which can beused for impregnation of the paper base.

While the paper can be rendered conductive and then dried, when it isused without drying, excess liquid is desirably expressed from theconductive paper before it is led into the electrocating bath.

The invention will be described in greater detail with reference to theaccompanying drawing in which the single figure is a diagrammaticsectional view depicting the process of the invention.

Referring more particularly to the drawing, ordinary paper is taken froma roll 10 and passed through a bath 11 containing surface active agent,the paper passing beneath the roller 12. The paper wet with nonaqueousmedium containing the surface active agent is then passed throughsqueeze rolls 13 to express excess liquid. The conductive paper soprovided is then passed to an electrocoating bath 14, the walls of whichform the anode of a unidirectional electrical system. A metal roller 15is positioned partially submerged within the bath 14, the roller 15being connected as cathode. The conductive paper is passed through thebath 14 while held on the surface of roller 15 by means of rolls 16 and17 to cause the electrodeposition of a photoconductive coating on theouter surface of the paper. The wet-coated paper is passed through adrying zone 18 and then wound on a wind-up roll 19.

The electrocoating bath in the invention is nonaqueous as previouslyindicated, and it is preferred that the liquid medium constituting thebulk of the bath be a hydrocarbon solvent which may be mineral spirits,naphtha, or an aromatic hydrocarbon such as toluene, xylene or a mixtureof the foregoing. While hydrocarbon solvents are preferred,oxygen-containing solvents can also be used such as ketones like methylethyl ketone, or esters such as butyl acetate. Halogen-containingsolvents such as trichloroethylene are also useful. The more activesolvents, such as the oxygen-containing solvents, are preferably used insmall amount to supplement the hydrocarbon solvents. When the solvent isvolatile or inflammable, the electrocoating bath 14 is covered with acover 20 to reduce loss of material and to maintain the concentration ofthe solvent vapors in the air in the vicinity of the electrodes (wherearcing may occur) above the upper limit of solvent-air mixtures whichcan burn.

The photoconductive material is zinc oxide, the merits of which are wellrecognized in the field. The zinc oxide is very finely divided, and canbe generally defined as being of pigment grade. The use of zinc oxide inphotoconductive coatings is well known and the same grades of materialuseful for this known purpose are useful herein.

In order to hold the deposited zinc oxide particles together, a resinousbinder is used. The specific nature of the resinous binder is of someimportance since it has been found that the presence of a smallproportion of carboxyl group is highly beneficial in enhancing adhesionof the zinc oxide coating to the paper base. Also, thecarboxylfunctional resins appear to wet the zinc oxide better whichhelps to cause the resin to migrate with the zinc oxide particles towardthe cathode. However, resins devoid of functional groups are alsouseful.

It is desired to stress that the use of carboxyl functional resins in anelectrocoating system in which the resin is to deposit at the cathode ofthe system is surprising since, in aqueous medium, acidic resins arewell known to deposit at the anode. It would seem that in the nonaqueoussystem of the invention ionization of the carboxyl groups does not takeplace to any controlling extent so that the primary factors influencingresin movement is association with the dispersed zinc oxide particleswhich migrate to the cathode.

Broadly, resins having an acid number of from O to about 60 may be used,but it is preferred to employ resins of lower acid number, e.g., below30. Acid numbers of from 630 are preferred, more preferably from 8 to20.

Referring more particularly to the resins which may be utilized, thelimiting factors are that the resin must possess significant solubilityin the nonaqueous solvent medium which is used and be capable of wettingthe zinc oxide in order to form a paste therewith. In order toillustrate resins devoid of functional groups which may be utilized inaccordance with the invention, reference is made to a copolymer ofstyrene and butadiene having a molecular Weight in the range of 400,000to 3 million, preferably from 750,000 to 1.5 million and which containsstyrene and butadiene in a weight ratio of 20/40. Up to about 8% byweight of fumaric acid may be included in the copolymer.

These resins are at least colloidally soluble in aromatic hydrocarbonsolvents such as xylene and toluene and preferably possess a glasstransition temperature range of from 20 C. to C. Glass transitiontemperature in the systems under consideration represents a meaningfulfactor since it is preferred that the resin which may exist in thesolution in colloidal form be capable of flowing to bind the zinc oxideparticles to the paper base even when no heat is used. On the otherhand, heat can be used to dry the coated paper and this permits higherglass transition temperatures to be used.

The zinc oxide pigment is simply ground into the resin solution in orderto form a paste in which resin is associated with the pigment forelectrical transport therewith. While the pigment to binder ratio issubject to variation, it is preferred to provide an initialsolvent-based paste containing zinc oxide and resinous binder in aweight ratio of from about 5:1 to 5:4, the paste being diluted withadditional solvent to provide the dilution which is desired. Theproportion of resin over that used in the initial paste is thenincreased and additional resin is added before, during or after solventdilution. Thus, the final electrocoating bath will contain a pigment tobinder ratio of from about 2:1 to 1:4 and, for best results, the bathwill 4 contain a pigment to binder ratio of from about 1:1 to about 1:2.When the resin has an acid number of 6 or higher, it seems to becomebetter associated with the Zinc oxide and both deposition and adhesionare improved.

The concentration of the nonaqueous bath is of secondary consideration,the resin solids content being appropriately within the range of from1-25% by weight, preferably in the range of from 5-15 by weight.

The resinous binder which is used is preferably an addition polymer,especial reference being made to acrylic copolymers illustrated bycopolymers of ethyl acrylate and methyl methacrylate and preferablycontaining a preponderance of the acrylate component in order tominimize the glass transition temperature of the copolymer and thesecopolymers will desirably include a small proportion of unsaturated acidsuch as acrylic acid, methacrylic acid, crotonic acid or itaconic acidso as to provide the limited acid number referred to hereinbefore. Onthe other hand, functional groups may desirably be included in thecopolymer, especial reference being made to the hydroxy group which maybe incorporated by the presence of allyl alcohol, hydroxyethylmethacrylate or the like or which may be provided by reaction ofmonoepoxide such as propylene oxide with any excess acidity which may bepresent in the copolymer. Still other functional groups such as aminegroups are desirably present, e.g., the copolymer may include anethylenically unsaturated amine such as dimethyl amino ethylmethacrylate in an amount to provide from 2-25 equivalent percent ofamine.

Polyvinyl chloride copolymers are also useful, especially those whichinclude from 60-90% by weight of polyvinyl chloride, with the balance ofthe copolymer being ethylenically unsaturated material copolymerizabletherewith and especially vinyl acetate or ethylene. These copolymers mayinclude a small proportion of ethylenically unsaturated acid includingacrylic acid or maleic acid and typically possess a molecular weight inthe range of 100,000 to 700,000, e.g., 400,000.

The presence of hydroxy or amine functionality in the resin maycontribute to the desirability of its selection and other aspects ofresin selection may have significance, but oil-modified resins must beavoided since the oil component leads to discoloration which isinconsistent with the provision of useful coated paper.

The electrophotographic coatings of the invention preferably includedyes and sensitizers which enhance the sensitivity of the zinc oxide tobecome excited by the light exposure to which it is subjected and toenhance the capacity of the paper to respond to illumination in thevisible range. Conventional agents of this type are known and theseagents have been found to codeposit with the resin and binder. For bestresults, it is preferred to employ dye selection as noted below.

In the electrophoretically deposited coatings of the invention, it hasbeen found that the phthalein dyes are particularly effective, referencebeing made to 4,5'-dichlorofluorescein and to 2,7-dichlorofluorescein. Avery small amount of methanol (12% of the weight of the paste) ishelpful to disperse the dye in the paste.

The particular nature of the paper to be coated is of secondaryconsideration and the accompanying examples include the use of acommercially available conductive paper as well as ordinary bond writingpaper and newsprint.

The nature of the supporting cathode is of secondary significance, asteel roller being illustrative. Similarly, the anode can be constitutedby any metal, but it will be appreciated that the anode releases metalions to the nonaqueous bath. Nonetheless, iron-containing metals havebeen used successfully, and the use of zinc is particularlycontemplated.

It is contemplated that relatively high voltages of 1 to 20 kilovolts,preferably from 2-l0 kilovolts will be particularly employed from whichthe desirability of closing the tank as previously discussed will beselfevident. In connection with the electrical current used, it will beunderstood that a unidirectional electrical current is contemplated, aterm which is intended to include momentary reversals in current withthe deposition of coating occurring 'while the current is in a singledirection to deposit material at the cathode. Below 1 kilovolt,electrophoretic movement and deposition are unduly slow and, above 20kilovolts, arcing is encountered and deposition is excessive.

The invention is illustrated in the examples which follow.

EXAMPLE 1 Paste composition Parts by wt. Zinc oxide pigment 200Styrene-butadiene resin (25% solids solution in toluene)see Note 1 160Chlorinated paraffin (70% chlorine by weight) having a specific gravityof 1.65 and a Ball and Ring melting point of 100 C. Methyl Green .006Acridine Orange .006 3',3,5,5"-tetrabromophenol-sulfophthalein .006

Norm-Copelymer of styrene and butadiene in a weight: ratio of /40 havinga colecular weight of about 1,000,000, Pliolite S-5B (Goodyear Rubber &Tire Co.) illustrates a commercially available resin useful in the aboveexample in place of the copolymer noted.

The paste is formed by simply grinding the ingredients together toprovide a uniform paste having a viscosity of 28 seconds measured in aNo. l Zahn Cup.

Conductive paper of the type conventionally used in the production ofelectrophotographic paper is passed through an electrocoating bath onthe surface of a metal roller cathode with the bath containing a mixtureof 20 parts of the paste described above diluted with 80 parts oftoluene and sufiicient additional styrene-butadiene resin solution toprovide a pigment to binder weight ratio of 2:1 and with speed regulatedto cause the paper to be immersed in the bath for a period of from 1-5seconds. Various voltages in the range of 1-10 kv. are used fordeposition. The coated paper is then baked at 300 F. for a period offrom 2-4 minutes.

The coatings are satisfactory, though the adhesion is not as good asmight be desired and there is some cracking of the coating during thebake. On the other hand, the product is satisfactory and readable printsfrom a typewritten original can be made by sending the finished paperthrough a Bruning 2000 copier which is a conventional dry copierutilizing visible light and a lens system exposure. Adhesion, evennessof deposition and the tendency of the coating to crack are all improvedby increasing the proportion of resin in the mixture which is easilydone by simply adding resin solution to the diluted paste.

EXAMPLE 2 Example 1 is repeated to provide a comparable productpossessing improved adhesion by increasing the proportion of resin. Thisis done by adding additional styrenebutadiene resin to the paste priorto dilution with toluene (3 parts of additional resin solution beingadded to 20 parts of the paste).

EXAMPLE 3 Examples 1 and 2 are repeated utilizing, in place of the 1 60parts of styrene-butadiene resin, an acrylic copolymer containing ethylacrylate and methyl methacrylate in proportions providing a solutioncopolymer having a glass transition temperature of -10 C. together witha small proportion of acrylic acid providing an acid number in the rangeof 13-17. The resin is provided in resin solids xylene solution.Examples 1 and 2 are also altered in this example by using ordinary bondwriting paper and newsprint handled in the manner shown in the drawing.More particularly, these non-conductive papers are passed beneath roller12 through a bath of toluene containing by weight 4% of isopropanol and1% of nonyl benzyl triethyl ammonium chloride. The impregnated paper isthen passed through press rolls 13 before being passed through theelectrocoating bath.

The results in the present example are comparable to those reported inExamples 1 and 2, but the acidic resin exhibits a superior capacity tobind the zinc oxide particles to one another and to the base so thatbetter adhesion is obtained at any given ratio of pigment to hinder.

The zinc oxide pigment used in the foregoing examples is made using aFrench furnace in which high purity electrolytic zinc is melted andvaporized, and the vapors are burned to form zinc oxide particles havingan average specific surface diameter in the range of from 0.136 to 0.408micron.

The invention is defined in the claims which follow.

I claim:

1. A method for the production of electrophotographic paper comprisingpassing a conductive paper through a nonaqueous electrocoating bathhaving a solvent portion constituted essentially by a hydrocarbonsolvent medium, with said paper being rendered conductive by thepresence therein of a surface active agent which is substantiallyinsoluble in said hydrocarbon solvent medium, said conductive paperbeing interposed between the cathode and the anode of a unidirectionalelectrical system having a voltage of at least about 1,000 volts, saidnonaqueous bath containing photoconductive zinc oxide pigment dispersedtherein and an oil-free resinous binder therefor, said resinous binderbeing associated with said pigment in a pigment to hinder ratio of from2:1 to 1:4 for codeposition at the cathode.

2. A method as recited in claim 1 in which the weight ratio of pigmentto resinous binder in said bath is from about 1:1 to about 1:2.

3. A method as recited in claim 1 in which said resinous binder is anaddition polymer.

4. A method as recited in claim 1 in which said resinous binder is anaddition polymer containing carboxyl functionality providing an acidnumber of from 6-30.

5. A method as recited in claim 1 in which said electrocoating bathfurther includes a phthalein dye.

6. A method as recited in claim 5 in which said dye is4',5'-dichlorofluoresceiu.

7. A method as recited in claim 5 in which said dye is2',7'-dichlorofluorescein.

8. A method as recited in claim 1 in which said nonaqueous bath includeszinc oxide and resinous binder dispersed in the hydrocarbon solvent.

9. A method as recited in claim 1 in which said resinous binder is anacrylic coplymer.

10. A method as recited in claim 1 in which said resinous binder is acopolymer of styrene and butadiene.

11. A method as recited in claim 1 in which said conductive paper issupported on said cathode.

12. A method as recited in claim 1 in which said unidirectionalelectrical system is operated at a voltage in the range of from 1-20kilovolts.

13. A method as recited in claim 12 in which said bath is employed in aclosed chamber.

14. A method as recited in claim 1 in which said conductive paper isrendered conductive by the presence therein of cationic surface activeagent.

References Cited UNITED STATES PATENTS 2,898,279 8/1959 Metcalfe et al.204181 3,268,433 8/1966 Abere 204-481 HOWARD S. WILLIAMS, PrimaryExaminer

